The 1854 cholera outbreak is the example of a natural experiment discussed often by David A. Freedman, e.g. in Statistical Models: Theory and Practice (Cambridge University Press) [1], chapter 1.3 (pp. 6–9).
Angrist, Joshua D. (1990). "Lifetime Earnings and the Vietnam Draft Lottery: Evidence from Social Security Administrative Records". American Economic Review. 80 (3): 313–336. JSTOR2006669.